Anyone can submit screenshots unless he is one of the judges. I think it was pretty clear and this is why there is no any special note about this. I would say 3 shots/person and I will add this to contest page as well

For those of us on widescreen setups, what's the preferred resize route?
My stock desktop is 3360x1050, and sometimes extends to 5040x1050 (2-3x 3360x1050)
By resize route, I mean that the shrunk images to the max limits don't really show much useful detail.

robbat2 3360x1050 should be ok provided that you cant resize it. Picassa may also accept bigger screenshot but if the screenshot is way too big we will have to resize it before we put it on
http://www.gentoo.org/main/en/shots.xml